Confused on Due Date

So I have very irregular periods always have!  We found out we were pregnant around 5 weeks based off a ultrasound scan!  I then had a follow up ultrasound at 8 weeks ans 12 weeks as well and date stayed the same!  We have measured 2 weeks ahead since 18 weeks!  Now I am supposed to be 34 weeks and since week 28 we have been measuring 4 weeks ahead!  Dr will not change date but I just feel that my initial due date is just not right! Anyone had any experience with this?